Self oscillating power supply

ABSTRACT

An inexpensive highly reliable power supply for small-sized electronic devices, in which any high-withstand-voltage diode which has been used conventionally is not used and reverse leak currents are prevented. A series circuit of a primary-side coil (34) of a switching transformer and a switching element (35) is connected between both ends of an input power source (31) and a series circuit composed of a resistor (32) and a capacitor (33) is also connected between both ends of the power source (31). The connecting point between the resistor (32) and capacitor (33) is connected to one end of the control winding (37) of the switching transformer and the other end of the winding (37) of the transformer is connected to the control terminal of the switching element (35). A discharge circuit (40) which is driven by signals from the control winding (37) of the transformer is used as a means for discharging the capacitor (33).

This Application is a U.S. National Phase Application of PCTInternational Application PCT/JP96/01233.

TECHNICAL FIELD

The present invention relates to a power supply device used in variouselectronic appliances, and more particularly to a power supply deviceuseful as a non-contact power supply device used in small portableappliances, for example, cordless telephone, cellular telephone, PHS,video with camera, and personal computer.

BACKGROUND ART

In a power supply device generally known, an output is obtained at asecondary side by resonating the voltage of a primary side coil of aswitching transformer and a capacitor connected at its both ends.

As means for obtaining a stabilized output at the secondary side, acircuit composition for controlling the primary side, or a circuitcomposition for controlling the secondary side are used, among others.

First, as the means for controlling the primary side, FIG. 15 shows acircuit diagram of a conventional power supply device for controllingand stabilizing the on/off period of switching by installing a controlcircuit at the primary side, and feeding back the gate signal of theswitching element as means for stabilizing and oscillating, to theoutput of the switching element by an impedance circuit composed of aseries circuit of resistance and diode. According to the diagram, aninput power source 1 is a DC voltage rectified and smoothed from acommercial power source, and a series circuit of starting circuitcomposed of a resistance 2 and a capacitor 3 is connected to both endsof the input power source 1, and a series circuit of a primary side coil4 of switching transformer and switching element 5 is connected, and acapacitor 6 is connected to both ends of the primary side coil 4 of theswitching transformer.

Moreover, the junction of the resistance 2 and capacitor 3 is connectedto the drain of the switching element 5 through a series circuit of aresistance 7 and diode 8, and is further connected to the gate of theswitching element 5 through a control winding 9 of the switchingtransformer. A capacitor 11 is connected to both ends of a secondaryside coil 10 of the switching transformer, and a capacitor 13 isconnected through a diode 12, thereby obtaining an output at both endsof the capacitor 13. Incidentally, the load side after the secondaryside coil 10 of the switching transformer is separable, and an outputcan be obtained as required.

The operation of the conventional power supply device is describedbelow. First, when the input power source 1 is applied, a voltage startsto be charged into the capacitor 3 through the resistance 2. The voltageof the capacitor 3 is fed into the gate of the switching element 5through the control winding 9 of the switching transformer, and whenreaching the threshold voltage of the gate, the switching element 5begins to conduct. As a result, a voltage is induced in the controlwinding 9 of the switching transformer and the secondary side coil 10 ofthe switching transformer, and the voltage of the control winding 9 ofthe switching transformer elevates, and the gate voltage of theswitching element 5 is further increased, so that the switching element5 is completely turned on instantly by the positive feedback action.

Therefore, the current of the primary side coil 4 of the switchingtransformer, that is, the drain current of the switching element 5increases linearly, and the energy is accumulated in the primary sidecoil 4 of the switching transformer. As the switching element 5 iscompletely turned on, an impedance circuit 14 of resistance 7 and diode8 (or, an impedance circuit 15 shown in FIG. 15, instead of thisimpedance circuit 14) begins to discharge the voltage of the capacitor3, that is, the gate voltage of the switching element 5. By suchfeedback action, when the gate voltage of the switching element 5becomes lower than the threshold voltage, the switching element 5 issuddenly turned off.

As the switching element 5 is turned off, the voltage induced in theprimary side coil 4 of the switching transformer is inverted, andresonance with the capacitor 6 occurs at the same time. When thisresonance voltage is inverted again, it is driven to turn on theswitching element 5 again through the control winding 9 of the switchingtransformer. At the same time, at the secondary side, too, resonance ofthe secondary side coil 10 of the switching transformer and thecapacitor 11 occurs, and a DC output is supplied into the secondary sideload 16 by a rectifying and smoothing circuit of diode 12 and capacitor13.

Next, a prior art of controlling the secondary side output is describedin a circuit diagram in FIG. 16. According to the diagram, referencenumeral 20 denotes a primary side power supply unit, being composed of aDC input power source 21, a high frequency current generating circuit 22connected thereto, a primary side resonance capacitor 23, and a primaryside coil 24, and reference numeral 25 is a secondary side power source,which is provided in a separate housing from the primary side powersupply unit 20, being composed of a secondary side coil 26, a secondaryside resonance capacitor 27 connected to both ends of the secondary sidecoil 26, a secondary side rectifier 28, and an output capacitor 29having one end connected to the secondary side rectifier 28, and otherend connected to the secondary side coil 26, and moreover an outputstabilizing circuit 30 and output capacitor 29 are connected, and asecondary side load (not shown) is connected to this output stabilizingcircuit.

As described herein, since the prior art is intended to control eitherthe primary side or the secondary side, it is used not only as a powersupply device of a general electronic appliance, but also as anon-contact type power supply device having the primary side andsecondary side provided in different housings.

However, in the circuit composition in FIG. 15, among the aboveconventional constitutions, the diode 8 used for feedback is required tohave a high withstand voltage because a voltage in reverse direction isapplied due to resonance of the primary side coil 4 of the switchingtransformer when the switching element 5 is turned off. Moreover, sincethe impedance of the control circuit is very high, a significant effectmay be applied to the switching action of turn-on and turn-off of theswitching element 5 due to reverse leak current of the diode 8, and thediode 8 is required to have a very small reverse leak current. Yet, foroperation at high frequency of several hundreds of kHz, high frequencyswitching is required at the same time. The diode satisfying suchcharacteristic is very hard to manufacture and very high in cost.

In the circuit composition in FIG. 16, incidentally, in order to obtainan output of high precision, a larger power loss occurs in the outputstabilizing circuit 30.

It is hence an object of the invention to solve such problems andpresent a power supply device capable of obtaining a stable secondaryoutput efficiently.

DISCLOSURE OF THE INVENTION

To solve the problems, according to the power supply device of theinvention, for controlling the primary side, a series circuit of aprimary side coil of a switching transformer and a switching element isconnected to both ends of an input power source, a series circuit of aresistance and a capacitor is connected in the input power source, thejunction of the resistance and the capacitor is connected to one end ofthe control winding of the switching transformer, other end of thecontrol winding of the switching transformer is connected to a controlterminal of the switching element, and means for discharging thecapacitor is composed of a discharge circuit driven by the signal of thecontrol winding of the switching transformer, and in this constitution,without using the diode of high withstand voltage, a power supply deviceoperating stably without effect of reverse leak current is realized.

For controlling the secondary side, meanwhile, a series circuit of acapacitor and an impedance variable circuit is connected to both ends ofthe secondary side coil, an output detection circuit for detecting theoutput of the secondary side coil is provided, and the impedancevariable circuit is controlled by the output of the secondary side coil,and therefore this constitution realizes a power supply device capableof maintaining the secondary side output very stably againstfluctuations of input voltage or output state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit block diagram of an embodiment of a power supplydevice of the invention;

FIG. 2 is a circuit block diagram of a second embodiment;

FIG. 3 is a circuit block diagram of a third embodiment;

FIG. 4 is an output characteristic diagram of the power supply device ofthe invention;

FIG. 5 is an input voltage-output power characteristic diagram of thepower supply device of the invention;

FIG. 6 is a circuit block diagram of a fourth embodiment;

FIG. 7 is a circuit block diagram of essential parts of a fifthembodiment;

FIG. 8(a) is a circuit block diagram of a seventh embodiment;

FIG. 8(b) is a specific circuit block diagram of an impedance variablecircuit of the seventh embodiment;

FIG. 8(c) is another specific circuit block diagram of an impedancevariable circuit the seventh embodiment;

FIG. 8(d) is a specific circuit block diagram of an output detectingcircuit of the seventh embodiment;

FIG. 8(e) and (f) are specific circuit block diagrams of an outputdetecting circuit of the seventh embodiment;

FIG. 9(a) and (b) are output voltage-current characteristic diagrams ofthe power supply device in FIG. 8(a) with various impedances forvariable impedance circuit 78;

FIG. 10 is a circuit block diagram of an eight embodiment;

FIG. 11 is an output characteristic diagram of the eighth embodiment;

FIG. 12 is a circuit block diagram of a ninth embodiment;

FIG. 13 is an output characteristic diagram of the ninth embodiment;

FIG. 14 is a circuit block diagram of a tenth embodiment;

FIG. 15 is a circuit block diagram of a conventional power supplydevice; and

FIG. 16 is a circuit diagram of an other conventional power supplydevice.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

An embodiment of a power supply device of the invention is describedbelow while referring to FIG. 1.

In the diagram, a control circuit is provided at the primary side, andaccording to the diagram, an input power source 31 is a DC voltagerectified and smoothed from a commercial power source, a series circuitof a resistance 32 and a capacitor 33 is connected at both ends of thisinput power source 31, and a series circuit of primary side coil 34 ofswitching transformer and switching element 35 is connected, and acapacitor 36 is connected at both ends of the primary side coil 34 ofthe switching transformer.

The junction of the resistance 32 and capacitor 33 is connected to oneend of a control winding 37 of the switching transformer, and other endis connected to the gate of the switching element 35. A dischargecircuit 40 composed of a control transistor 38 and resistances 39a, 39bis driven by a signal of the control winding 37 of the switchingtransformer, thereby discharging the electric charge of the capacitor33. A capacitor 42 is connected to both ends of a secondary side coil 41of the switching transformer, and a capacitor 44 is connected through adiode 43, so that an output is obtained at both ends of the capacitor44. The load side after the secondary side coil 41 of the switchingtransformer is separable, so that an output may be obtained as required.

The operation is described below. When the input power source 31 isapplied, a voltage begins to be charged into the capacitor 33 throughthe resistance 32. The voltage of the capacitor 33 is fed into the gateof the switching element 35 through the control winding 37 of theswitching transformer, and when this voltage reaches the thresholdvoltage of the gate, the switching element 35 begins to conduct. As aresult, a voltage is induced in the control winding 37 of the switchingtransformer and the secondary side coil 41 of the switching transformer,and by voltage elevation of the control winding 37 of the switchingtransformer, the gate voltage of the switching element 35 is furtherincreased, and the switching element 35 is instantly turned oncompletely by the positive feedback action.

Accordingly, the current of the primary side coil 34 of the switchingtransformer, that is, the drain current of the switching element 35increases linearly, and energy is accumulated in the primary side coil34 of the switching transformer. At this time, when the voltage of thecontrol winding 37 of the switching transformer reaches the thresholdvoltage of the control transistor 38, the control transistor 38 isturned on instantly, and the capacitor 33 begins to be dischargedthrough the resistance 39b. By such feedback action, when the gatevoltage of the switching element 35 becomes lower than the thresholdvoltage, the switching element 35 is suddenly turned off (in theembodiment, since a field effect transistor (FET) is used in theswitching element 35, the threshold voltage is the gate cut-off voltage,and when transistor is used as this switching element, the base voltageof the transistor is the threshold voltage).

When the switching element 35 is turned off, the voltage induced in theprimary side coil 34 of the switching transformer is inverted, and atthe same time, resonance with the capacitor 36 occurs. When thisresonance voltage is inverted again, the switching element 35 is drivento be turned on again through the control winding 37 of the switchingtransformer. Same as in the prior art in FIG. 15, at the secondary side,too, resonance of the secondary side coil 41 of switching transformerand capacitor 42 occurs, and a DC output is supplied into a load 45through a rectifying and smoothing circuit of diode 43 and capacitor 42.

Embodiment 2

FIG. 2 is a circuit block diagram of other embodiment, and what differsfrom the embodiment in FIG. 1 is that a clamp circuit 48 composed of aresistance 46 and a diode 47 is added. Unnecessary spike voltageoccurring in the control winding 37 of the switching transformer isapplied to the gate of the switching element 35, and adverse effects maybe caused on the turn-on and turn-off action of switching. Hence, thevoltage applied to the gate is clamped by the clamp circuit 48 byforward voltage drop (VF) of the diode 47, thereby removing theunnecessary spike voltage.

Herein, the element used in the clamp circuit 48 for correcting thetemperature changes of the threshold value of the gate voltage of theswitching element 35 may be either one diode or plural diodes, or azener diode or a circuit combining them.

In the foregoing embodiments, the output is determined by the on/offperiod of the switching action. As mentioned above, the ON period is thetime from the switching element 35 being turned on until the voltage ofthe capacitor 33 is discharged through the resistance 39b to be lowerthan the threshold voltage of the switching element 35. The OFF periodis the time from the switching element 35 being turned off until thevoltage of the capacitor 33 is charged from the input power source 31through the resistance 32 to reach the threshold voltage of theswitching element 35. Therefore, it is known that the output isdetermined by the charge and discharge time of the capacitor 33. It ishence possible to set at an arbitrary output by using a variableresistor in the resistance 32 for charging and discharging the capacitor33, or the resistance 39b.

Thus, according to the embodiments, one end of the control winding ofthe switching transformer is connected to the junction of the seriescircuit of the resistance and capacitor connected to the input powersource, the other end of the control winding is connected to the controlterminal of the switching element, and a discharge circuit for drivingby the signal of the control winding of the switching transformer isconnected to one end of the control winding, and therefore, whencomposing the oscillation circuit and control circuit at the primaryside, diode of high withstand voltage is not necessary, and there is noeffect of reverse leak current, and the operation is stable, so that analmost ideal switching action is effected, and a highly reliable powersupply device can be realized by using inexpensive parts.

Embodiment 3

FIG. 3 is a circuit block diagram of a different embodiment, in which aninput power source 51 is a DC voltage rectified and smoothed fromcommercial power source, or a DC power source such as car battery, andat both ends of the input power source 51, a resistance 52 and acapacitor 53 are connected in series, and a resistance 54 is connectedparallel to the capacitor 53. Moreover, at both ends of the input powersource, a series circuit of a primary side coil 55 of switchingtransformer and a switching element 56 is connected, and a capacitor 57is connected at both ends of the primary side coil 55 of the switchingtransformer.

The junction of the resistance 52 and capacitor 53 is connected to oneend of the control winding 58 of the switching transformer, and theother end of the control winding 58 is connected to the gate of theswitching element 56.

A series circuit of resistance 59 and resistance 60 is connected betweenthe drain and source of the switching element 56, a peak voltage controlcircuit 64 is composed by connecting a transistor 62 and a resistance 63from the junction of the resistance 59 and resistance 60 through aconstant voltage diode 61, and the collector of the transistor 62 isconnected to the junction of the resistance 52 and capacitor 53. At bothends of the secondary side coil 65 of the switching transformer, acapacitor 66 is connected, and a capacitor 68 is connected through adiode 67, and an output is obtained at both ends of the capacitor 68.Incidentally, the load after the secondary side coil 65 of the switchingtransformer is separable, and an output may be obtained as required.

Describing the operation, first, when the input power source 51 isapplied, a voltage begins to be charged in the capacitor 53 through theresistance 52. The voltage of the capacitor 53 is fed into the gate ofthe switching element 56 through the control winding 58 of the switchingtransformer, and when this voltage reaches the threshold voltage of thegate, the switching element 56 begins to conduct. As a result, a voltageis induced in the control winding 58 of the switching transformer andthe secondary side coil 65 of the switching transformer, and by voltageelevation of the control winding 58 of the switching transformer, thegate voltage of the switching element 56 is further increased, and theswitching element 56 is instantly turned on completely by the positivefeedback action.

Accordingly, the current of the primary side coil 55 of the switchingtransformer, that is, the drain current of the switching element 56increases linearly, and energy is accumulated in the primary side coil55 of the switching transformer. At this time, when the voltage of thecapacitor 53 is fixed at a certain voltage by voltage dividing by theresistance 52 and resistance 54, and the gate voltage of the switchingelement 56 is also limited at this voltage. Therefore, due to thecharacteristic of FET, by the limitation of the gate voltage, the draincurrent is also limited, and, as a result, the voltage between the drainand source is raised. Hence, the voltage of the primary side coil 55 ofthe switching transformer is decreased, and at the same time the voltageof the control winding 58 of the switching transformer is decreased, andthereby the gate voltage of the switching element 56 decreases untilbecoming lower than the threshold voltage, so that the switching element56 is suddenly turned off. In this embodiment, meanwhile, since thefield effect transistor (FET) is used in the switching element 56, thethreshold voltage is a gate cut-off voltage, but when a bipolartransistor is used as the switching element, the base voltage becomesthe threshold voltage.

When the switching element 56 is turned off, the voltage induced in theprimary side coil 55 of the switching transformer is inverted, andresonance is induced with the capacitor 57 at the same time. At thistime, the drain-source voltage of the switching element 56 is elevatedin a sinusoidal form by the resonance phenomenon. Supposing the peakvalue of the drain-source voltage of the switching element 56 to be Vp,when becoming ##EQU1## where R13 is the resistance value of theresistance 59, and R16 is the resistance value of the resistance 60, apeak voltage control circuit 64 is put in action to effect negativefeedback control to lower the voltage of the capacitor 53, therebycontrolling so that Vp may be constant in every pulse of switching. Soonthis resonance voltage is inverted again, and the switching element 56is controlled to be turned on again through the control winding 58 ofthe switching transformer. At this time, at the secondary side, too,resonance of the secondary coil 65 of the switching transformer andcapacitor 66 occurs, and a DC output is supplied to the secondary sideload from the rectifying and smoothing circuit of the diode 67 andcapacitor 66.

Hence, the peak value Vp of the resonance voltage is controlled to bealways constant in every pulse of switching in spite of fluctuations ofthe input power source 51. Accordingly, the output voltage generated inthe secondary side coil 65 of the switching transformer is alwaysconstant as shown in FIG. 4, and an extremely stable voltage may besupplied to the secondary side output.

FIG. 5 is a characteristic diagram showing the relation between theinput voltage and output voltage, which shows that the output power isalways kept constant as being controlled by the peak voltage controlcircuit 64.

Embodiment 4

FIG. 6 is a circuit block diagram of an embodiment of the invention,being a developed example of the embodiment in FIG. 3, and explainingonly the difference from the embodiment in FIG. 3, in constitution, theoutput of the peak voltage control circuit 64, that is, the collector ofthe transistor 62 is directly connected to the gate of the switchingelement 56, and the same effects as in the embodiment in FIG. 3 areobtained.

Embodiment 5

FIG. 7 is a circuit block diagram of a peak voltage control circuitwhich is an essential component in an embodiment of the invention, andother circuit parts are same as in the circuit composition in FIG. 3.

Instead of the transistor 62 and constant voltage diode 61 in FIG. 3, acomparator (or operational amplifier) 69 and a reference voltage 70 areused to compose a peak voltage control circuit 64a, and the same effectsas in the embodiment in FIG. 3 are obtained. In the foregoingembodiments, the control circuit is provided at the primary side, and anembodiment of installing the control circuit at the secondary side isdescribed below.

Embodiment 7

FIG. 8(a) is a circuit block diagram of an embodiment of a power supplydevice having a control circuit provided at the secondary side, in whichreference numeral 71 is a primary side power source unit composed of aDC input power source 72, a high frequency current generator 73, aprimary side coil 74, and a primary side resonance capacitor 75, 76 is asecondary side power source unit composed of a secondary side coil 77, asecondary side resonance capacitor 78, an impedance variable circuit 79,a secondary side rectifier 80, and output capacitor 81, and an outputdetection circuit 82, and the impedance variable circuit 79 is insertedbetween the secondary side resonance capacitor 78 and the secondary sidecoil 77, and it is designed to be controlled by the output detectioncircuit 82 connected to both ends of the output capacitor 81.

In thus constituted non-contact type DC power supply device, theoperation is described below.

In the primary side coil 74, a high frequency current generated in thehigh frequency current generator 73 flows, and by this current, a highfrequency voltage is generated in the primary side coil 74. This highfrequency voltage is a sinusoidal wave owing to the resonance phenomenonof the primary side resonance capacitor 75 and the inductance of theprimary side coil 74.

In the secondary side coil 77, a voltage waveform similar to thissinusoidal wave is generated, but the portion corresponding to its halfwavelength is blocked by the secondary side rectifier 80. The blockedelectric power of the half wavelength portion is once accumulated in thesecondary side resonance capacitor 78, and is transmitted to the outputat the time of next oscillation period. The characteristic of the outputvoltage and current at this time is shown in FIG. 9(a).

When the impedance is inserted in series to the secondary side resonancecapacitor 78, the characteristic of the output voltage and current ischanged by the impedance as shown in FIG. 9(b). Therefore, the outputvoltage or current is detected by the output detection circuit 82, andthe impedance of the impedance variable circuit 79 is controlled so thatit may be constant, and therefore the electric power accumulated in thesecondary side resonance capacitor 78 is adjusted, and the output can becontrolled at high precision.

FIG. 8(b) and (c) show practical examples of the impedance variablecircuit 79, (b) showing a parallel circuit of transistor 83 and diode 84and (c) using a field effect transistor 85, and FIG. 8(d), (e), (f) showpractical examples of the output detection circuit 82.

FIG. 8(d) is composed of a transistor 86, and resistances 87, 88 forfeeding partial voltage of the output voltage to its base, (e) using adifferential amplifier 89, and (f) uses an output current detectioncircuit for detecting the output current of a transistor 90.

In the diagram, (A) denotes a terminal unit for receiving an output forcontrol from an output voltage (current) detection circuit to theimpedance variable circuit 79, and (B) shows an output terminal unit ofan output voltage (current) detection circuit to the impedance variablecircuit 79.

Reference numerals 91, 92, 93 are resistances, and 89a is a referencebattery for generating a reference voltage.

In this embodiment, therefore, by providing the secondary side resonancecapacitor 78 with impedance variable circuit 79 and output detectioncircuit 82, the output, which was hard to stabilize hitherto, isstabilized, and a power supply device obtaining an output of highprecision can be realized.

Embodiment 8

FIG. 10 is a circuit diagram of an embodiment of the invention, in whichthe primary side composes a constant power oscillating circuitstabilized at the primary side, by connecting a primary side coil 98 ofswitching transformer to an AC power source 94 through a rectifying andsmoothing circuit consisting of a rectifying circuit 95 and a capacitor96, and a primary switching element 97, and connecting a control circuit99 to the primary switching element 97. At the secondary side of theswitching transformer, a transistor (FET) 101 is connected to asecondary side coil 100 through a capacitor C1, and a capacitor 103, aresistance 104, and an output terminal 105 are connected to thesecondary side coil 100 of the switching transformer through a diode102. Moreover, output terminal 105 is connected to a transistor 106 andthe transistor (FET) 101 through the resistance 104, the transistor 106is connected from the base to a detection resistance 108 through aresistance 107, and to an output terminal 110 and a transistor 109, andthe transistor (FET) 101 is connected in series to the capacitor C1, andthe impedance of the transistor (FET)101 is varied, and thereby theenergy transmitted to the load side is changed, so that a constantvoltage and constant current control is achieved.

In this constitution, the voltage obtained through the secondary sidecoil 100 of the switching transformer and the capacitor C1 is rectifiedand smoothed by the diode 102 and capacitor 103, and this output isconnected to the output terminals 105, 110 through the detectionresistance 108, so that the current may be always stabilized. A batteryand others are connected to an external load 111, and the state of theexternal load (battery, etc.)111 is monitored by an output currentchangeover circuit 112, and, as shown in FIG. 11, the transistor 109 isdriven, and the charging current supplied into the external load 111 ischanged over to rapid charge level or trickle charge level, so that theexternal load 111 may be charged optimally.

Embodiment 9

FIG. 12 shows other embodiment of the invention, which is improved fromthe embodiment in FIG. 10.

In the embodiment in FIG. 10, constant voltage and current control isrealized by varying the impedance of the transistor (FET)101, and inrapid charge, the transistor (FET) 101 is completely turned on, and amaximum output is obtained, so that a maximum power is supplied to theexternal load 111. In an output characteristic diagram in FIG. 11,corresponding to curve f and curve g, it is charged by curve i (rapidcharge region). At this time, the transistor 101 is turned on, and theimpedance between the drain and source is small, and heat generation iskept low.

In trickle charge, however, when the external load 111 is fully charged,the transistor 109 is turned off by the output current changeovercircuit 112, and a current flows into the detection resistance 108, andwhen the voltage at its both ends reaches the threshold voltage of thetransistor 106, the transistor 106 is turned on and the transistor (FET)101 is turned off, and the output for limiting the output currentbecomes a constant current drooping characteristic. Hence, it is chargedby curve h (trickle charge region) in the output characteristic diagramin FIG. 11. At this time, the transistor (FET) 101 operates in an activeregion, and the impedance between the drain and source increases, andthe heat generation of the transistor (FET) 101 is very large, and thepower consumption increases, and this problem is solved by thisembodiment.

Explaining only the difference between this embodiment and FIG. 10,instead of the capacitor C1, a first capacitor C2, and a series circuitof transistor (FET) 101 and second capacitor C3 parallel thereto areprovided between the both ends of the secondary side coil 100, and thesum of the capacities of the first and second capacitors is nearly sameas the capacity of the capacity C1 in FIG. 10.

In this constitution, too, the voltage obtained by the secondary sidecoil 100 of the switching transformer, and first capacitor C2 and secondcapacitor C3 is rectified and smoothed by the diode 102 and capacitor103, and this output is connected to the output terminals 105, 110through the detection resistance 108, and the current is alwaysstabilized, and same as in the embodiment in FIG. 10, in rapid charge,the transistor (FET) 101 is completely turned on and a maximum output isobtained, and a maximum power is supplied to the external load 111. Inan output characteristic diagram in FIG. 13, corresponding to curve aand curve b, it is charged by curve e (rapid charge region). At thistime, since the transistor (FET) 101 is turned on, the impedance betweenthe drain and source is very small, and heat generation is kept low.

In trickle charge, moreover, when the battery of the external load 111is fully charged, the transistor 109 is turned off by the output currentchangeover circuit 112, and a current flows into the detectionresistance 108, and the voltage at both ends reaches the thresholdvoltage of the transistor 106, thereby turning on the transistor 106 andturning off the transistor (FET) 101, so the output current is limitedand becomes a constant current characteristic, so that it is charged bycurve c (trickle charge region) in the output characteristic diagram inFIG. 13. At this time, since the transistor (FET) 101 operates in anactive region, the impedance between the drain and source is large, andheat generation of the transistor (FET) 101 is very large, but since thecapacitors C2 and C3 are provided parallel, the capacity of prior artC1=C2+C3, the currents flowing in the capacitors are distributed into i2and i3, and the current flowing in each capacitor is a high frequencycurrent, and hence it is determined by the switching frequency and thecapacity of the capacitor, and therefore the current is large when theswitching frequency is high and the capacity of the capacitor is large,but when the impedance between the drain and source of the transistor(FET) 101 is constant, the relation between the current i1 flowing inthe capacitor C1 in FIG. 10 and the current i3 flowing in the secondcapacitor C3 in the embodiment is i1>i3, so that the heat generation ofthe transistor (FET) 101 in this circuit is kept low.

Moreover, since the dynamic range of the output is small herein,considering the load curve (curve d in FIG. 13) by the first capacitorC2, the capacitors C2 and C3 are set appropriately so as to maintain thetrickle charge region (curve c in FIG. 13).

Embodiment 10

FIG. 14 is a circuit block diagram of a different embodiment, in whichan input power source 113 is a DC voltage rectified and smoothed from acommercial power source, and a parallel circuit of primary side coil 115and capacitor 116 is connected through a switching unit 114 to compose aprimary side power supply device.

A secondary side power supply device 118 has a capacitor 120a and aswitching element 120 connected in series to both ends of a secondaryside coil 119, and a series circuit of diode 121 and capacitor 122 isconnected, and an output is supplied to the load through a pulse widthcontrol unit 123, a constant voltage control unit 124, and a constantcurrent control unit 125.

The operation and the circuit composition are specifically describedbelow. In the primary side power supply device 117, when the input powersource 113 is applied, the switching unit 114 operates, and a highfrequency current is supplied into the primary side coil 115. At thesame time, the primary side coil 115 resonates with the capacitor 116,and a high frequency voltage is induced.

Moreover, the voltage induced in the primary side coil 115 is furtherinduced in the confronting secondary side coil 119, and when theswitching element 120 is turned off, the secondary side coil 119 andcapacitor 120a do not resonate, and only the voltage induced from theprimary side coil 115 is obtained, and when the switching element 120 isturned on, the secondary side coil 119 and capacitor 120a resonate, anda high output is obtained. Still more, the resonated voltage andnon-resonated voltage are averaged by the time ratio of ON period andOFF period, and rectified and smoothed by the diode 121 and capacitor122, so that an output is obtained at both ends of the capacitor 122.

Furthermore, in order that the voltage obtained in the capacitor 122 maybe always constant, a signal is transmitted from the constant voltagecontrol unit 124 to the pulse width control unit 123. Herein, the pulsewidth control unit 123 is controlled by a constant frequency, and whenthe output voltage is lowered, the ON period of the output pulse isextended, so that it is controlled to extend the ON period of theswitching element 120, that is, the ON period of the capacitor 120a, andthe resonance period of the secondary side coil 119 and capacitor 120ais extended, thereby acting to raise the voltage.

To the contrary, when the output voltage is raised, it is controlled toshorten the ON period of the output pulse, and the resonance period ofthe secondary side coil 119 and capacitor 120a is shortened, therebyacting to lower the voltage.

Also, in order that the current supplied to the load may be alwaysconstant, a signal is transmitted from the constant current control unit125 to the pulse width control unit 123, and when the output currentdecreases, the ON period of the output pulse is extended to control toextend the ON period of the switching element 120, that is, the ONperiod of the capacitor 120a, and the resonance period of the secondaryside coil 119 and capacitor 120a is extended, thereby acting to increasethe current.

To the contrary, when the output current increases, it is controlled toshorten the ON period, and the resonance period of the secondary sidecoil 119 and capacitor 120a is shortened, thereby acting to decrease thecurrent.

Thus, by controlling the on/off period of the switching pulse by thepulse width control unit 123, the on/off period of the capacitor 120a iscontrolled, and the output voltage and output current is controlled, sothat a constant voltage and constant current output is supplied to theload.

Meanwhile, only one of the constant voltage control unit 124 andconstant current control unit 125 may be used, depending on whether theload requires a constant voltage or a constant current.

Thus, in the invention, at the secondary side, the first capacitor andswitching element are connected in series to both ends of the coil, andthe second capacitor is connected from the junction of the coil and thefirst capacitor through the diode, and either the constant voltagecontrol unit or the constant current control unit is provided, and alsothe pulse width control unit for on/off control of the switching elementby the signal from the constant voltage control unit or constant currentcontrol unit is provided, and therefore the output is obtained by on/offcontrol of resonance of the secondary side coil 119 and capacitor 120a,not depending on analog control as in the transistor (FET) 101 in FIG.12, and heat generation is very small, and it contributes much todownsizing of the device.

INDUSTRIAL APPLICABILITY

Thus, the power supply device of the invention is a power supply devicecapable of obtaining a stable output by controlling the primary side orcontrolling the secondary side.

In particular,

(1) In an aspect in which one end of a control winding of a switchingtransformer is connected to a junction of a series circuit of aresistance and a capacitor connected to an input power source, other endof the control winding is connected to a control terminal of a switchingelement, and a discharge circuit driven by a signal of the controlwinding of the switching transformer is connected to the one end of thecontrol winding, when composing an oscillation circuit and a controlcircuit at the primary side, high withstand voltage diodes are notnecessary, there is no effect of reverse leak current and the operationis stabilized, and a nearly ideal switching operation is effected, sothat a highly reliable power supply device is realized by usinginexpensive parts.

(2) In an aspect in which a clamp circuit is provided, by clamping thevoltage applied to the gate of the switching element, unnecessary spikevoltage can be removed.

(3) In an aspect in which a primary side coil of switching transformerand a switching element are connected in series to both ends of an inputpower source, a first resistance and a capacitor are connected in seriesto the input power source, a second resistance is connected parallel tothe capacitor, the junction of the first resistance and the capacitor isconnected to one end of a control winding of the switching transformer,other end of the control winding of the switching transformer isconnected to a control terminal of the switching element, and a peakvoltage control circuit, as means for discharging said capacitor,composed of a voltage detection unit for detecting the peak voltage bydividing the resistance from the drain of the switching element and acontrol unit composed of transistor and resistance through a constantvoltage diode from the junction divided by the resistance is provided asmeans for discharging the capacitor, when composing an oscillationcircuit and a control circuit at the primary side, each pulse ofswitching can be controlled, and precise and stable action is realized,so that nearly ideal switching characteristic and output characteristicmay be obtained.

(4) In a power supply device in which a primary side coil supplied witha high frequency current, and a secondary side coil installed in acasing different from the primary side coil confront each other, and anelectric power is transmitted from the primary side coil to thesecondary side coil, a series circuit of capacitor and impedancevariable circuit connected to both ends of the secondary side coil, andoutput detecting means for detecting the output by the secondary sidecoil are provided, and in an aspect for controlling the impedancevariable circuit by output of the output detecting means, by controllingthe impedance of the impedance variable circuit, the electric poweraccumulated in the secondary side resonance capacitor is adjusted, sothat the output may be controlled precisely.

(5) In an aspect in which the output detecting means is connected byinstalling an output current changeover circuit connected to the outputdetecting means, the charging current can be changed over between rapidcharge and trickle charge (ordinary charge).

(6) In an aspect in which other capacitor is connected parallel to aseries circuit of a capacitor and a transistor which is an impedancevariable circuit connected to both ends of a secondary side coil, heatgeneration in trickle charge can be suppressed, and hence cooling plateis not needed, and the transistor can be reduced in size, contributingto downsizing of the power supply device.

(7) In an aspect in which a first capacitor and a switching element areconnected in series to both ends of a coil at the secondary side, asecond capacitor is connected from the junction of the coil and firstcapacitor through a diode, a constant voltage control unit or a constantcurrent control unit is provided, and a pulse width control unit foron/off control of the switching element by a signal from the constantvoltage control unit or constant current control unit is provided, sincean output is obtained by on/off control of resonance of the secondaryside coil and capacitor, not depending on analog control, heatgeneration is small, and it contributes to downsizing of the device.

(8) In an aspect in which a primary side coil and a secondary side coilare provided in different housings, it is useful for portable electronicappliances such as cordless telephone, as a non-contact type powersupply device.

We claim:
 1. A power supply device, wherein one end of a primary sidecoil of a switching transformer is connected to a first terminal of aninput power source, other end of this primary side coil is connected toone end of a switching element, other end of this switching element isconnected to a second terminal of the input power source, further oneend of a first resistance is connected to the first terminal of saidinput power source, other end of this first resistance is connected toone end of a capacitor, other end of this capacitor is connected to thesecond terminal of said input power source, the junction of said firstresistance and said capacitor is connected to one end of a controlwinding of said switching transformer, other end of the control windingof said switching transformer is connected to a control terminal of saidswitching element, a series circuit of a second resistance and a controlelement is connected between the second terminal of said input powersource and the control terminal of said switching element, and a controlterminal of said control element is connected to the junction of saidfirst resistance and said capacitor, whereby the primary side coil ofsaid switching transformer and the control winding is a positivefeedback through said switching element, and the control element isconducted by elevation of the voltage of the control winding of saidswitching transformer over a specified value to control the voltage atboth ends of said capacitor.
 2. A power supply device of claim 1,further comprising a clamp circuit composed of a resistance connectedbetween one end of the control winding of said switching transformer andthe control terminal of said switching element, and one or a pluralityof diodes of same polarity connected in series, connected between thecontrol terminal of said switching element and the other end of thecontrol winding of said switching transformer.
 3. A power supply device,wherein one end of a primary side coil of a switching transformer isconnected to a first terminal of an input power source, other end ofthis primary side coil is connected to one end of a switching element,other end of this switching element is connected to a second terminal ofthe input power source, further one end of a first resistance isconnected to the first terminal of said input power source, other end ofthis first resistance is connected to one end of a capacitor, other endof this capacitor is connected to the second terminal of said inputpower source, a second resistance is connected parallel to saidcapacitor, the junction of said first resistance and said capacitor isconnected to one end of a control winding of said switching transformer,other end of the control winding of said switching transformer isconnected to a control terminal of said switching element, a peakvoltage control circuit is provided as means for controlling the voltageof the capacitor for dividing and detecting the peak voltage between thejunction of the end of the primary side coil of the switchingtransformer and one end of the switching element and the second terminalof the input power source, and applying to the control terminal of thecontrol element when exceeding the specified voltage, the primary sidecoil of the switching transformer and the control winding are connectedso as to be a positive feedback through the switching element, and thevoltage of the capacitor is discharged by the control output of the peakvoltage control circuit.
 4. A power supply device comprising a primaryside coil supplied with a high frequency current and a secondary sidecoil installed in a housing different from said primary side coil,confronting each other, for transmitting an electric power from saidprimary side to said secondary side coil, further comprising a seriescircuit of a capacitor connected at both ends of said secondary sidecoil and an impedance variable circuit, and output detecting means fordetecting an output voltage or an output current by said secondary sidecoil, wherein said impedance variable circuit is controlled to increasethe impedance of said impedance variable circuit when the output voltageof said secondary side coil is raised by the output of said outputdetecting means.
 5. A power supply device of claim 4, wherein an outputcurrent changeover circuit is connected to the output detecting means tocontrol said output detecting means.
 6. A power supply device of claim5, wherein other capacitor is connected parallel to said series circuitof a capacitor and an impedance variable circuit connected to both endsof a secondary side coil.
 7. A power supply device comprising a primaryside coil supplied with a high frequency current, and a secondary sidecoil confronting said primary side coil, for transmitting an electricpower from said primary side coil to said secondary side coil, wherein afirst capacitor and a switching element are connected in series to bothends of said secondary side coil, a second capacitor is connected fromthe junction of said secondary side coil and said first capacitorthrough a diode, constant voltage control means for detecting thevoltage of this second capacitor and/or constant current control meansfor detecting the output current from said second capacitor to the loadis provided, and pulse width control means for receiving the output ofsaid constant voltage control means and/or said current control meansand controlling said switching element by the pulse of specifiedfrequency is provided, whereby when the constant voltage control mansdetected decline of the output voltage, the ON period of the outputpulse is extended, and when rise of the output voltage is detected, theON period of the output pulse is shortened, and when the constantcurrent control means detects decrease of the output current, the ONperiod of the output pulse is extended, and when increase of the outputcurrent is detected, the ON period of the output pulse is shortened. 8.A power supply device of claim 1, wherein a primary side coil and asecondary side coil are provided in different housings.
 9. A powersupply device of claim 3, wherein a primary side coil and a secondaryside coil are provided in different housings.
 10. A power supply deviceof claim 4, wherein a primary side coil and a secondary side coil areprovided in different housings.
 11. A power supply device of claim 7,wherein a primary side coil and a secondary side coil are provided indifferent housings.